Personal computers have their obvious benefits, but they also can be big power users. According to one extreme hypothetical example proposed by computer expert Michael Bluejay, a computer could consume 1,752 kilowatt-hours per year, costing more than $300 in electricity bills.

With that in mind, what is the best way to save energy when you're not using your computer? Should you turn it off, or is sleep mode good enough? How much energy does a screensaver save? And aside from electricity usage, what is optimal for the functioning of the computer?

Understanding Different Energy Saving Modes

"'Sleep' and 'standby' reduce energy consumption while not powering down completely," explains Jordana Viuker, an energy-efficiency program manager for a California-based energy consulting firm. "'Hibernate' saves your current session as it is and then actually shuts down the computer, saving even more energy. But for maximum efficiency, when you're not using your computer, it is best to turn it off," Ms. Viuker advises, lest it continue to draw energy. What? A computer draws energy even when turned off?

It's true, even when turned off, computers (like most other electrical devices) continue to draw "phantom" energy. Says Ms. Viuker, “leaving any electrical appliance plugged in when it's in the power 'off' position—whether it be your laptop, desktop, or for that matter, television -- uses 'phantom' electricity.” What to do? “To avoid 'phantom' energy usage, turn off and unplug all appliances, including computers, when not in use.”

Power Use in Standby or Sleep Mode

As Michael Bluejay points out, the 0-6 watts used in these modes is insignificant compared to electrical energy used for heating, cooling and lighting, but when you have a house full of electronic devices each drawing small amounts of energy, the costs add up. He suggests that if you want to feel good about saving energy with your computer, you should at least set it to sleep automatically (with the monitor off) after 15-20 minutes. As for the screensaver, Bluejay makes the excellent point that a monitor running a screensaver is a monitor burning energy, between 17-80 watts. Your best option is to just turn it off and unplug it when not in use.

There is also an old debate about whether it is better for extending the life of a computer to leave it on all the time, or to turn it on and off regularly. The Lawrence Berkeley National Laboratory says that modern hard drives are not significantly affected by frequent shut-downs (compared to older models) and that it may actually increase the lifetime of the device by powering down on nights and weekends.

Although it is anecdotal, I can say from personal experience that I still use a laptop I've had since 2002, almost 10 years. The only hardware that has failed is the battery, and otherwise it runs reasonably well and has been shut down and turned on many times over the past decade.